1. Field of the Invention:
This invention relates generally to coffee makers and more particularly has reference to a drip-type coffee maker in which a brewing and dispensing unit makes coffee and discharges it into a decanter sitting on a warming plate.
2. Description of the Prior Art:
Existing coffee makers normally use decanters formed entirely of glass or metal. Those materials have a high heat transfer coefficient which causes a substantial heat loss through the walls of the decanter. As a result of such high heat losses, it is necessary to maintain a relatively high temperature on the warming element to maintain the coffee at a sufficiently high temperature to please the palate of coffee drinkers. Plastic, having a lower heat transfer coefficient is a more desirable material to minimize heat loss through the side walls of the decanter but suffers the shortcoming that the rate of heat transferred through the bottom thereof from a relatively flat warming plate is also minimized. Such plastic decanters, thus exhibiting such a high resistance to heat transfer through the bottom wall that the temperature required on the warming plate to maintain the coffee at the desired temperature could be so great as to melt or deform the plastic decanter. Also, heating of the coffee immediately adjacent the bottom wall to such high temperatures, even in the localized area, has a severe detrimental effect on the coffee taste. Thus, there exists a need for a coffee warming well and low coefficient of heat transfer plastic decanter which cooperate to provide for efficient heat transfer therebetween.
In addition, most existing decanters are designed to be uncovered when being filled and when storing a quantity of warm coffee. A substantial amount of heat is lost to air currents circulating across the opening in the top of the decanter. The losses increase when the coffee maker is exposed to air currents in an air conditioned room or office building.
It is common practice to manufacture coffee machines with warming plates which are formed with upturned peripheral rims to retain the coffee decanter generally centered thereon and to act as a cooling fan to dissipate heat to the atmosphere to maintain such peripheral walls at a relatively cool temperature. However, such plates are normally fairly shallow and are not designed to afford a sufficiently close fit with the decanter to produce a sufficiently high rate of heat exchange therebetween to enable use of plastic in the decanter.
The air currents also pass over the warming plate, which typically is a flush mounted flat unit or, at the most, a slightly dished out warming plate loosely receiving the bottom of the decanter. This causes additional heat loss from the bottom of the decanter and from the surface of the warming plate itself and also reduces the efficiency of heat transfer from the warming plate to the decanter.
Coffee must be kept at some optimum temperature, typically about a 180.degree. F., in order to have the proper flavor and taste. Because of the heat losses and inefficient heat transfer which occur in existing coffee makers, the warming plate must often be kept at a temperature of about 350.degree.-400.degree. F. to maintain the coffee at the desired temperature. This produces a hot spot at the bottom of the decanter which overheats the coffee in that region. The resulting deterioration in flavor becomes pervasive as the coffee circulates on thermal currents throughout the decanter.
The hot spot problem is exacerbated when the warming plate is formed of material having a relatively low thermal conductivity. Heat does not distribute evenly across the bottom of the decanter. Rather, it builds up in the regions which are directly above the heating coils under the warming plate. The resulting heat gradient further deteriorates the flavor of the coffee.
Efforts to solve this overheating problems has led to the proposal of coffee warmers incorporating a water reservoir heating to about 200.degree. F. However, use of glass decanter with such warmers resulted in dissipation of heat to the atmosphere at such a great rate that the temperature of the coffee fell below the palatable level.
Glass decanters also pose a significant safety problem. When an empty glass decanter is left on the warming plate, the bottom heats rapidly causing thermal expansion and loss of temper. The weakened bottom cam break off at that time or, more importantly, at some later time when the decanter is filled. Anyone present when the break occurs could be cut by broken glass or burned by scalding hot coffee.
In effort to solve this safety problem, decanters have been fabricated with a plastic upper portion and a thin walled metal lower portion. While eliminating some of the danger associated with glass decanters, the thin metal wall actually accelerates the heat transfer causing rapid deterioration of the coffee flavor. Heretofore, it was not generally recommended that plastic decanters be placed on warming plates because it was believed that the bottom would melt when placed on a warming plate having a sufficiently high temperature to, considering the relatively low heat transfer coefficient of plastic, maintain the body of coffee at the desired temperature for tasty drinking.
Coffee decanters made of plastic material have been known for use in microwave coffee makers, but there the selection of materials is dictated by a requirement for microwave transparency, not for their thermal insulative characteristics.